Distribution: Is known only from two localities in the Sierra de Coneto, a volcanic mountain located in central Durango, México, on the western boundaries of the Chihuahuan Desert. Sierra de Coneto constitutes the southeastern extreme of the Sierra de Promontorio and the eastern limit of an endorrheic (closed) basin (Laguna de Santiaguillo). The climate in the area is temperate semi-dry climate. The predominant substratum is formed by ignimbrites (igneous extrusive rocks of acidic pH). Megacorax gracielanus has been collected between 24°55'49" to 25°5'11" N, and 104°47'5" to 104°57'10" W, at 2210 and 2360 m. It occurs in colonies and it is scarce to abundant in patches on rocky areas with acidic soils. The dominant vegetation is open xerophytic woodland of Pinus cembroides Zucc. (pinyon pine) with Quercus emoryi Torr. and sclerophyllous shrubs, mainly Arctostaphylos pungens Kunth and Quercus depressipes Trel. The two known populations are about 18 Km apart (air distance). Based on a survey in June 2002 (SG and LL), it is very likely that Megacorax gracielanus grows in much of the intervening area, as well as in other parts of the Sierra de Coneto. It has not been possible to survey these areas because there are no roads. The new survey showed that at the type locality, Megacorax gracielanus is much more abundant than originally thought, and in some areas is one of the dominant elements in the lower stratum. It grows along an open canyon about 1 km long (at least) and 150 m wide. In some places there are 4-5 plants/square meter. In the other collection site (southernmost) Megacorax gracielanus is scarce and infrequent with only a few plants found (< 20). But it is interesting that in both areas it grows not only under shrubs and trees, but also in open areas along recently open roads, on fragmented rocks. It is much more abundant and well developed under the shrubs but it seems to adapt well to colonizing disturbed areas, which raises the question of why this unique plant is not much more widely distributed. The Sierra de Coneto is also the only known locality for Mammillaria theresae Cutak., and perhaps the perceived localized nature of their distribution is due to the lack of thorough collecting in this region of México.
Habit: Xeromorphic, rhizomatous, weakly woody shrub from a branched, ascending caudex up to 1.4 cm thick. Stems profusely branched, ascending, (15-)25-60 cm high, covered with thin gray bark and exfoliating in longitudinal strips on the lowermost stems, becoming pale brown to yellow and shredding or flaky on stems further up, the bark of main stem and primary branches smooth and pale brown, upper branches terete or angled, yellow to purple or red tinged, puberulent with patent or ascending, translucent hairs 0.07-0.2(-0.3) mm long.
Leaves: Stipules 0.2-1 mm long, linear to subulate, puberulent to glabrate, pale yellow, persistent at least briefly after the leaf falls. Leaves alternate, usually reduced and crowded on short lateral branches and thus appearing fasciculate, young ones thick and somewhat fleshy, 8-12 x 0.8-1.3 mm, linear to linear-lanceolate, green to reddish or purple, minutely papillate, often with minute white dots on the upper surface, the midrib broad and evident on the lower surface except toward the apex, narrow and impressed on the upper surface, otherwise the veins indistinct, puberulent with translucent hairs ca. 0.1-0.2 mm long, becoming glabrate, margin entire to shallowly sinuato-denticulate, whole leaf revolute or sometimes just the margins revolute, apex obtuse to subacute, base gradually attenuate, sessile on a thickened base that persist as a protuberant scar.
Flowers: Flowers in the axils of the upper leaves, the branches resembling leafy, many-flowered racemes, both chasmogamous and cleistogamous flowers present on the same branch, the floral parts variable in size, especially of cleistogamous ones, pedicels up to 14 mm long, filiform, ascending to erect, purple, puberulent with white hairs 0.1-0.25 mm long, or sometimes glabrate, floral tube inconspicuous, 0.2-0.6 mm long, enlarged by annular disk in upper 2/3, puberulent, buds up to 6.2 mm long, narrowly elliptic, the free tips 0.4-0.5 mm long, linear, sepals regular and equal, valvate in aestivation, reflexed in anthesis, rarely two of the sepals coherent, (1.2-)3.4-6.2 x 0.3-0.8 mm, oblong-lanceolate, deep red or reddish purple, sparsely appressed to spreading puberulent on adaxial surface, especially toward the apex, puberulent on abaxial surface, abruptly narrowed at the apex, petals white, (1.3-)3.8-6.2 x 1.2—2.6 mm, narrowly obovate, rarely with inconspicuous purple veins, shorter than or equaling the sepals, rounded to acute at apex, tapering to the base, in chasmogamous flowers appearing radially arranged in dry specimens, but displaced to one side of the flower in anthesis. Stamens all fertile, 8(6), in two unequal series, the antisepalous ones longer, filaments the longer set 1.2-4 mm long, the shorter set 0.9-2.4 mm long, with smaller anthers, pink to purple at least toward the proximal half, anthers of longer stamens 0.6-0.7 mm long, those of shorter stamens 0.4-0.5 mm long, broadly oblong, cream-colored, aging to yellow, ovary 1.9-2.1 x 0.9-1 mm, turbinate or cylindric, gradually tapering to the base, puberulent, style filiform, 1.8-8 mm long, pink to purple, held above the stamens or surrounded by stamens, stigma entire, capitate, ca. 0.4-0.6 mm in diameter, whitish.
Fruit: Capsule (3-)4.4-5.8(-6.7) x 1.3-1.8 mm, thin-walled, ellipsoid-cylindrical, oblong-cylindrical, nearly terete or walls slightly distended by seeds, tapering at base, truncate at summit, pink to purple, puberulent to glabrescent, 3-4 celled and 3-4-valved, the valves dehiscing 1/8 to nearly 1/2, spreading in maturity, the thin partitions between the locules persisting to maturity, with several seeds in 1 irregular row in each cell.
Seeds: Seeds 1-1.2 x 0.4-0.55 mm, narrowly obovoid or narrowly oblong, pale brown, glossy, the abaxial side convex, the raphal surface (adaxial) side slightly depressed with margins slightly incurved, with raphe visible as a longitudinal ridge, the testa appearing nearly smooth, but in fact minutely pitted-reticulate(>50x).
Notes: Apparently self-compatible judging from the presence of cleistogamous flowers.